[SyncFS] Rollback function from version 4 to version 3.

chrome/browser/sync_file_system/drive_backend/metadata_db_migration_util.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_METADATA_DB_MIGRATION_UTIL_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_METADATA_DB_MIGRATION_UTIL_H_
 
 #include <string>
 
 #include "chrome/browser/sync_file_system/sync_file_system.pb.h"
@@ skipping 40 matching lines @@
 //   "folder:yyy"  =>  "yyy"
 //   "zzz"         =>  "zzz"
 std::string RemoveWapiIdPrefix(const std::string& resource_id);
 
 // Migrate |db| schema from version 0 to version 1.
 SyncStatusCode MigrateDatabaseFromV0ToV1(leveldb::DB* db);
 
 // Migrate |db| schema from version 1 to version 2.
 SyncStatusCode MigrateDatabaseFromV1ToV2(leveldb::DB* db);
 
+// Rollback |db| schema from version 4 to version 3.
+SyncStatusCode MigrateDatabaseFromV4ToV3(leveldb::DB* db);
+
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_METADATA_DB_MIGRATION_UTIL_H_